Your ISP is "throttling" P2P connections. This is getting very common.
They use hardware boxes and software that analyzes traffic by port #,packet info ,or by IP adresses.
So,you can have a 3 Mbps pipe and surf the web fast, but P2P sppeds are drastically reduced, or "throttled"
you can tell if
you start d/ling a file, it's cruising along nicely, and then after 1-2 minutes, the speeds drops to 0 or close to it. I though it was always the other guy's fault until I started looking into this and read up on Throttling.
Which ISP do you use? Go look for a forum about it on dslreports.com and confirm this with other users.